The invention relates generally to superconducting magnet systems and more particularly to superconducting magnets operating in an alternating current (AC) environment.
Exemplary superconducting magnet systems operating in an AC environment include a transformer, a generator, a motor, superconducting magnet energy storage (SMES), and a magnetic resonance (MR) system. Although a conventional MR magnet operates in a DC mode, some MR magnets may operate under an AC magnetic field from the gradient coils when the gradient leakage field to the magnet is high. Such an AC magnetic field generates AC losses in the magnet. An illustrative discussion of exemplary details of the MR system is presented, for explanatory purposes.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In one example of an MR system, a cold mass comprises a superconducting magnet, a magnet coil support structure, and a helium vessel. Liquid helium contained in the helium vessel provides cooling for the superconducting magnet and maintains the superconducting magnet at a low temperature for superconducting operations, as will be understood by those skilled in the art. The liquid helium maintains the superconducting magnet approximately and/or substantially at the liquid helium temperature of 4.2 Kelvin (K). For thermal isolation, the helium vessel that contains the liquid helium in one example comprises a pressure vessel inside a vacuum vessel.
The cold mass in one example comprises relatively large and/or extensive metal components. The helium vessel comprises relatively large pieces of stainless steel and/or aluminum. The magnet coil support structure comprises composite materials and/or relatively extensive distributions of metal.
When the superconducting magnet for the MR system operates in an AC field environment, eddy current is induced in the metal of the cold mass. Eddy currents are induced in a relatively large metal component of the helium vessel. In a further example, eddy currents are induced in a relatively extensive metal component of the magnet coil support structure.
The eddy currents generate heat. The heat generated by the eddy currents adds to the heat that needs to be dissipated for operation of the MR system. The eddy currents represent AC losses for the MR system, since the superconducting magnet needs to be maintained at the low temperature for the superconducting operations.
It would therefore be desirable to promote a reduction in presence and/or extent of metal available for eddy currents and resultant AC losses in a superconducting magnet system. To promote heat removal from the superconducting magnet, it would be desirable to promote liquid helium cooling flow and avoidance of helium vapor lock in a superconducting magnet system.